1

Membrane reactor operating at high

selectivity and substantially increased

productivity for the oxidative

dehydrogenation of ethane

Andre C. van Veen

The University of Warwick - School of Engineering

Coventry CV4 7AL

andre.vanveen@warwick.ac.uk

Outline

Conclusions and perspectives.

Performance in C2H6 activation over structured catalysts

Process intensification aims on enhancing rates at least tenfold

introducing suitable chemical engineering measures.

Development of a laboratory membrane reactor.

The example of a catalytic membrane reactor illustrates how

employing structured reactors can enable substantial

performance gains operating “a different kind of catalysis”.

Motivation for oxidative dehydrogenation of ethane.

2

2

Some examples of structured reactors

Micro-structured devices:

Structured reactors might lift many constraints in heterogeneous

catalysis

Monolith reactors

However, catalyst stability remains the key in “intensified” systems.

3

Interdisciplinary approach

Production

with low CO2

footprint

Development of

high temperature

reactors

Access to

commodities and

bulk chemical

Activation of

light alkanes

4

3

Upgrading of ethane to ethylene

5

Context:

Shale gas brings access to C2+ hydrocarbons at sites remote from

current cracker infrastructure.

Challenge:

Suitable catalysts, reactor and process concepts need

development.

Approach:

Implementation of oxidative dehydrogenation of ethane as

continuous smaller scale source of ethylene.

Inconvenience:

Conventional steam cracking is energy intensive and optimized

for very high capacities.

[Review: C.A. Gärtner et al., ChemCatChem 5 (2013), p. 3196.]

Low temperature catalyst system

Mo

VTe

NbO

x c

ata

lysts

(M

1 p

ha

se

)

6

Hydrothemal

synthesis,

175°C

Mixing

at 80°C

Aqueous

solutions of

precursor

salts

Mixed

metal

oxide

Filtration

Calcination

MoVTeNbOx

T = 370 °C

T = 400 °C

0

10

20

30

40

50

60

0 10 20 30 40 50 60

Ethane conversion [%]

Eth

en

e y

ield

[%

]

♦ ▲

• The oxidative dehydrogenation of ethane with molecular oxygen over

MoVTeNbOx proceeds at high selectivity

• What are the catalyst properties leading to the outstanding performance?

[D. Hartmann et al., 15th ICC (2012), Poster 2.02_8004 / Talk PS.02.]

4

Catalytic activity of the M1 phase M

oV

Te

NbO

x c

ata

lysts

(M

1 p

ha

se

)

7

• Direct dependence of the rate

constant on the M1 phase

concentration through origin

• k1 represents the rate constant

of ethene formation

• T = 370 °C

• Catalytic activity originates solely from M1 phase

• Literature advances amorphous layers as active sites,

but this must then be linearly related to the M1 phase

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200

M1

co

nc

en

tra

tio

n /

%k1 / µmol g-1s-1

C2H6

C2H4

COx

Reaction network

k1

k2

k3

Apparent activation energy

Mo

VTe

NbO

x c

ata

lysts

(M

1 p

ha

se

)

8

• Apparent activation energy

for ethene formation is

constant for various

catalysts providing

different rates

• Ea = 80 ± 10 kJ/mol

• k1 represents the rate

constant of ethene

formation 0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200

Ap

pa

ren

t E

a/ k

J m

ol-

1

k1 / µmol g-1s-1

• Similar M1 surface properties for all samples

• Upper temperature limit for reaction imposed by

catalyst temperature stability (critical above 450°C)

5

Supported alkali chloride catalysts S

olid

co

re / m

olte

n s

he

ll ca

taly

sts

9

LiKCl

MgO

MgO/Dy2O3

Solid oxide core Overlayer (molten under

reaction conditions)

• 1 mol% Dy2O3 in the support increases

the activity.

• The chloride overlayer is vital for the

activity and the selectivity.

2 C2H6 + O2 → 2 C2H4 + 2 H2O

Influence of the overlayer thickness

So

lid c

ore

/ m

olte

n s

he

ll ca

taly

sts

10

• Conversion reaches

maximum at 10 wt% Cl-

and decreases with

increasing chloride

loading.

• Ethene selectivity is

enhanced by the presence

of chlorides, but changes

only modestly afterwards.

Overlayer plays an

important role for

reaction.

Decreasing activity with

increasing layer thickness

suggests transport influence.

LiKCl/MgDyO, T=600°C, WHSV = 0.8 h-1, pges = 1 bar , pEthane = pO2 = 70 mbar

S(C2H4)

X(C2H6)

6

Reaction pathways for chloride catalysts S

olid

co

re / m

olte

n s

he

ll ca

taly

sts

11

MgO

MgO/Dy2O3

• Support has unselective sites and needs to be fully covered to achieve high

selectivity.

• Chloride phase and/or interface stabilize and stores the oxidizing species.

• Ethane is not absorbed by the chloride layer, thus ODH step is assumed to

take place on the surface.

LiCl

LiKCl

LiNaCl

O2

[O2] [O*]

C2H6

C2H4

H2O Intermediate

formation

Intermediate

performs ODH

Spatial separation of oxidation

and reduction sites.

Molten salt shell acts like an

oxygen membrane

Catalytic membranes for oxidations

Ca

taly

tic m

em

bra

ne

s in

se

lective

oxid

atio

n

12

Context:

Atom efficient upgrading of light hydrocarbons could

provide alternative feed-stocks for tomorrows chemistry.

Challenge:

Need for membrane materials, reactor configurations, new

catalysts and their thin film coating.

Approach:

Dense catalytic membrane reactors suppress gas-phase

oxygen enabling high catalyst selectivity and efficiency.

Inconvenience:

Selective catalysts in fixed bed oxidative dehydrogenation

(ODH) show reasonably low reaction rates.

7

Transport through MIEC membranes C

ata

lytic m

em

bra

ne

s in s

ele

ctive

oxid

atio

n

13

L

Porous membranes (at high T): “simple” viscous flow, Knudsen diffusion

Dense membranes: Complex scheme requiring multifunctional materials

Each step might be a ”bottle neck”, Surface bulk transport kinetics.

Thus, one needs to obtain information on each specific step.

1. Adsorption of molecular oxygen

2. Dissociation ?

3. Formation of oxygen ions

4. Diffusion of oxygen ions and

electrons through the membrane

5. Formation of adsorbed oxygen

6. Recombination ?

7. Desorption of molecular oxygen

Permeation flux measurement

Ca

taly

tic m

em

bra

ne

s in

se

lective

oxid

atio

n

14

He

He + O2

Air

N2 (+ O2)

0 5 10 15 20 25 30 35

1.0

1.5

2.0

973

1023

1073

1023

973

973

1048

1073

973

T / K

J(O

2)

/ m

l m

in-1cm

-2

t / d

Highest permeance at 1023 K reported so far

Oxygen flux is not stable below 1023 K

Feed Side : F(O2) = 10 ml min-1, F(N2) = 40 ml min-1

Permeate Side : F(He) = 20 ml min-1

1mm thick Ba0.5Sr0.5Co0.8Fe0.2Ox membrane

[A.C. van Veen et al., Chem. Commun. 2003 (2003), p. 32.]

8

Oxidative dehydrogenation of C2H6 C

ata

lytic m

em

bra

ne

s in s

ele

ctive

oxid

atio

n

15

Reactor Ionic oxygen permeation and reaction

O2- O2-

e- e-

membrane

ca

taly

st la

ye

r

O2- O2- Ionic oxygen diffusion

through membrane

Release of

ethylene

Release of water

h° h°

VO°° VO°°

h° h°

VO°° VO°°

e- e-

Air side Ethane side

O-

O-

C-H bond activation and

ethane adsorption

x

OO

x

OO

2 C2H6 + O2 → 2 C2H4 + 2 H2O

Control of oxygen species and availability enhances selectivity.

C2H6 + He

Catalytic modification by V/MgO

Ca

taly

tic m

em

bra

ne

s in

se

lective

oxid

atio

n

16

Catalyst inventory approximately 1mg per cm² of membrane.

10 m

After V/MgO deposition and testing

10 m

SEM image of bare membrane

• Specific area (50 m²/g at 1075K)

• Well dispersed particles (coverage 40 %)

• Preparation by adapted Sol-Gel synthesis and spin coating

• Direct coating of the membrane ensures best possible interface

9

Catalytic modification by Pd C

ata

lytic m

em

bra

ne

s in s

ele

ctive

oxid

atio

n

17

Catalyst inventory approximately 1 μg per cm² of membrane.

Ar

Laser YAG :

Nd

Metal rod

Powder or

membrane

Aggregates

He

• Coverage 10 %

• Well dispersed particles

• Homogenous particles size

• Laser vaporization deposits metallic nano clusters

25 nm

TEM image of Pd deposits

(carbon replica image)

Use in oxidative dehydrogenation of C2H6

Ca

taly

tic m

em

bra

ne

s in

se

lective

oxid

atio

n

18

Clear impact of catalytic modification on ODHE performance.

960 1000 1040 1080 1120

20

40

60

80

100

V/MgO (1)

V/MgO (2)

Pd

no modif.

X / %

T / K

960 1000 1040 1080 112020

40

60

80

100

V/MgO (1)

V/MgO (2)

Pd

no modif.

S / %

T / K

Reaction conditions:

Air compartment: Fair=50mL/min, pair=1.2bars

C2H6 compartment: FC2H6/He=37mL/min, pC2H6=0.25bars

10

Use in oxidative dehydrogenation of C2H6 C

ata

lytic m

em

bra

ne

s in s

ele

ctive

oxid

atio

n

19

Membranes combine advantages of ODHE and steam-cracking.

No coke formation.

No limitation by equilibrium.

No flammability issues.

Ethylene production by oxidative dehydrogenation of ethane

(ODHE) is desirable, but high yields are economically vital.

950 1000 1050 1100 1150

10

20

30

40

50

60

70

80

Catalyst coating

V/MgO

Pd

No modifi.

Y(C

2H

4)

/ %

T / K[M. Rebeilleau et al., Catal. Today. 104

(2005), p. 131.]

Oxidative coupling of CH4

Ca

taly

tic m

em

bra

ne

s in

se

lective

oxid

atio

n

20

Reactor

2 CH4 + O2 → C2H4 + 2 H2O

Minimal presence of oxygen in gas-phase enhances selectivity.

Ionic oxygen permeation and reaction

O2- O2-

e- e-

membrane

ca

taly

st la

ye

r

O2- O2- Ionic oxygen diffusion

through membrane

Gas-phase

coupling

Dehydrogenation

h° h°

VO°° VO°°

h° h°

VO°° VO°°

e- e-

Air side Methane side

O-

O-

C-H bond activation and

methyl radical formation

x

OO

x

OO

11

Use in the oxidative coupling of CH4 C

ata

lytic m

em

bra

ne

s in s

ele

ctive

oxid

atio

n

21

High permeability:

1100 1150 1200 1250 1300

0

1

2

3

4

5

6

7

8

9

BSCFO membrane

(wash-coat)

(gel)

(bare)

C2 s

ele

ctivity /

%

T / K

1100 1150 1200 1250 1300

0

10

20

30

40

50

60

BSMFO membrane

(bare)

(gel)

(wash-coat)

C2 s

ele

ctivity /

%

T / K

Low permeability:

Probing the

membrane

action (using

Pt/MgO model

catalysts).

Optimized

BSCFO /

catalyst:

C2-yield up to

17%.

[L. Oliver et al., Catal. Today. 142 (2009), p. 34.]

[S. Haag et al., Catal. Today. 127 (2007), p. 157.]

Dense membranes for selective oxidation

Ca

taly

tic m

em

bra

ne

s in

se

lective

oxid

atio

n

22

Control the oxygen permeation flux

Lower working temperature

Improved catalyst-membrane interface

Increase scalability of the technology (new membranes)

No operational costs for oxygen separation

Smaller integrated systems (multifunctional reactor)

Green processes due to higher selectivity to target products

Increased security by avoiding the use of gas-phase oxygen

Research subjects:

Identified benefit:

12

Conclusions

New structuring concepts could enable

economically attractive performances in

alkane activation (supporting technology).

Mechanistic understanding is a prerequisite to design

structured short contact time reactors.

Oxidation catalysis can be enhanced by dense membrane

technology opening new operational windows.

Structured reactors / catalysts enable outstanding

performances when mutual development occurs.

23

Perspectives

Diversification of catalyst

structuring compulsory to open

new technology to more target

reactions.

Exploration of alkane activation

with optimized catalysts (oxidative

coupling).

24

13

Acknowledgements

Helpful discussions:

Y. Schuurman, C. Mirodatos,

J. Lercher

PhD students:

M. Rebeilleau, L. Olivier, D. Hartmann, C. Gärtner

Financial support and material supply:

Acenet project AL2OL,

EU Projekts Cermox and Topcombi, Solvay S.A.

25

Many thanks for your attention